SUNSPOTS AND EARTHQUAKES

Pahiatua Herald, Volume XLVIII, Issue 14066, 13 February 1939, Page 3

SUNSPOTS AND EARTHQUAKES

RELATION OUTLINED EFFECT OX “CYCLES.” The earthquake tells a great deal about the earth, reserving only that amount itself which is more interesting than anything else, namely, when the next earthquake will occur, writes E. S. Grew, in the Observer, London. About the'place where it. will happen it is less reticent. Many years of the accountancy of earthquakes which John Milne set in motion have yielded precise information about the zones all over the globe where the most violent of them all will have their centres.

The earthquakes themselves have been made to tell by the measuring instruments set up in the world’s necklace of observatories where in the depths of the earth’s crust some breakdown has set them in motion, and how big the break is. These are the first steps on the long road to the knowledge of when and where the next breakdown will take place.

They do not seem to carry us very far, but some progress has been made.

EARTHQUAKE “SWARMS.” At present the only forecast of a strong earthquake to come is afforded by the occurrence of what are called earthquake swarms, comparatively inferior disturbances which have been found to precede some of the disastrous earthquakes of Japan. These swarms have sometimes gone on for several years beforehand, and point to the slow accumulation of strains in the crust which will at last declare themselves in the collapse of some faultily balanced crustal arch of rock. Dr. Charles Davison, who for 50 years of his long life has been one of the accountants of earthquakes, employs a less empirical method of forecasting, though the periodicities he endeavours to establish are of a very modest kind. He begins with an assumption something like that which explains earthquake swarms. When the earth’s crust is about to give way, a slight force acting in one direction may start it, or acting in the opposite direction may stop it or delay it. If the magnitude of this outside force is itself subject to periodic variation, then the variations may be impressed on, or reflected by, the frequency of earthquakes in an earthquake zone. Not on all earthquakes, but on those which lend themselves to observation and classification.

The classification of earthquakes divides them into those which shake the whole earth and those which disturb only a part of it, but there are a number of subdivisions. Destructive earthquakes are of three grades —those which destroy towns, those which break down bridges, and those which, leaving the roofs intact, crack walls and ceilings. Dr. Davison’s records show that for all-world earthquakes, registered at the seismo-logic-al observatories, as well as for grc at earthquakes, the most likely month of the year is July. “MILD - EARTHQUAKES.

Il the earthquakes are further classified so as to take in those who' exhibit themselves first on land, or arise under the sea ; or those which begin in the Northern or Southern Hemisphere, then mid-June to the end ot Julv is the epoch claiming the. larger ones, and the end of May or September the ordinary or less spectacular ones.

But, except in China the epochs of earthquakes of the highest and most destructive intensity usually fall in winter. December in the Northern Hemisphere and the end of June m the Southern.

Soifie exceptions offer themselves for examination of criticism, especial) v among tin' mild earthquakes, of wlifcli neglecting the 60,01)') nervou* quivers agitating the earth’s skin. tl>< re are yet hundreds every year. These? as well as the world’s seawave earthquakes Iwhich arc not tidal waves) most often fa.ll in October; and the aeep focus earthquakes which originate lower down in the earth’s crust tha-n the granite layer of 12 t-j IS miles thickness, and sometimes far down in the basaltic foundations. « xhibit a preference for the end of May. Dr. Davison then examines wlmt arc the possible outside causes whicli might account for this periodiev. Enlarging the scope ol hi s periodi-

cities at least as far as thev will legitimately go. he further examines an 11 -year period corresponding to the ascertained sunspot cycle ; and a 19-vear period closely connected with the nutation of arth. EFFECT OF RAINFALL

By nutation, which is another word for nodding, is meant the wobbling of the earth’s axis which is revealed by the movement of the pole as it describes an irregular circle in 19 years. .Such a. movement might well be expected to put strains on the interior of the earth whicli would reveal themselves in the earth’s crust at more or less regular intervals as they accumulated.

The v await further examination of the earthquake records. The 11-year the 580 years from 1305 to 1888 there have been 55 11-yoar periods in which any influence exerted would be the same for all the earth. The records offered bv the chroniclers of China. and Japan exhibit what may he taken as a periodicity of notable earthquakes disastrous or destructive, corresponding roughly with the appearance of the greater a Teas of. sunspots. Dr. Davison’s interpretation is that the sunspot cycle- corresponds to an increase of earth rainfall when the sunspots are at their maximum: and that this additional fall of moisture facilitates movements of the earth’s crust in either direction.